ELECTRONIC DEVICES
Assist. prof. Laura-Nicoleta IVANCIU, Ph.D.
C9 – Summing and differential
amplifiers with OpAmp
2
Laura-Nicoleta IVANCIU,
Electronic devices
Contents
Summing amplifiers with OpAmp
Inverting summing amplifier
Non-inverting summing amplifier
Differential amplifiers with OpAmp
Recap –circuits with OpAmp
C9 – Summing and differential amplifs w/ OpAmp
3
Laura-Nicoleta IVANCIU,
Electronic devices
C9 – Summing and differential amplifs w/ OpAmp
Types of amplifiers with OpAmp
NI
NI II Amplifier
vIground non-inverting
ground vIinverting
vI1 vI2 differential
vI1, vI2 ground summing, non-inverting
ground vI1, vI2 summing, inverting
C8
C8
C9
4
Laura-Nicoleta IVANCIU,
Electronic devices
Summing amplifiers
Inverting summing amplifier
C9 – Summing and differential amplifs w/ OpAmp
+−= 2
2
1
1IIO v
R
R
v
R
R
v
RRR 2
21 ==
Relationship between resistors to obtain the
average of input voltages:
How can we compute vO?
+VPS
-VPS
5
Laura-Nicoleta IVANCIU,
Electronic devices
Summing amplifiers
Inverting summing amplifier
C9 – Summing and differential amplifs w/ OpAmp
Example
a)
vO
(
vi
1,
vi
2) assuming op amp in the active region. What is the application of the
circuit?
b) Considering
vi
1 = 2 V, plot the VTC
vO
(
vi
2) for
vi
2∈[-5 V; 5 V]. What is the
vi
2
range, so that the amplifier works in its active region?
c) Plot
vI
1(
t
),
vI
2(
t
) and
vO
(
t
) for
vI
1(
t
)= 1sinωt [V],
vI
2(
t
)= 0.5sinωt [V].
d) Resize
R
1,
R
2,
R
3,
R
4so that
vO
= -(
vi
1+
vi
2).
e) Modify the circuit, in order to obtain a non-inverting summing circuit, with
vO
=
vi
1+ v
i
2.
6
Laura-Nicoleta IVANCIU,
Electronic devices
Summing amplifiers
Non-inverting summing amplifier
C9 – Summing and differential amplifs w/ OpAmp
+
+
+
+= 2
21
1
1
21
2
3
4
1IIO v
RR
R
v
RR
R
R
R
v
21 IIO vvv +=
Relationship between resistors to have ?
4321 RRRR ===
4321 and RRRR ==
Usually
7
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
How can we compute vO?
2
1
2
1
1
2
43
41IIO v
R
R
v
R
R
RR
R
v−
+
+
=
8
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
1
1
2
43
4
11IO v
R
R
RR
R
v
+
+
=
2
1
2
2IO v
R
R
v−=
Superposition method
9
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
2
1
2
1
1
2
43
41IIO v
R
R
v
R
R
RR
R
v−
+
+
=
If the goal is to amplify (vI1-vI2):
1
2
1
2
43
41R
R
R
R
RR
R=
+
+
4
3
2
1R
R
R
R=
( )
21
1
2IIO vv
R
R
v−=
The circuit amplifies the difference between the input voltages and rejects
common mode signals.
In practical situations: R1 = R3 and R2 = R4
For vI1 = vI2, vO= 0.
noiseIvvv += 11
noiseI
vvv +=
22
)()( 2121 vvAvvvvAv vnoisenoisevO −=−−+=
10
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
Superposition method
431 RRRI+=
Input resistance, seen by vI1
12 RRI=
Input resistance, seen by vI2
11
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
Example
A sensor provides a variable signal, vi, with a dc component, VI.
It is necessary to amplify the variable signal, that carries
information, 10 times.
Design a differential amplifier for this requirement.
12
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
Example
REFIO V
R
R
tv
R
R
RR
R
tv
1
2
1
2
43
4)(1)( −
+
+
=
( )( )
IiIO V
R
R
tvV
R
R
RR
R
tv
1
2
1
2
43
41)( −+
+
+
=
01
1
2
1
2
43
4=−
+
+R
R
R
R
RR
R
101
1
2
43
4=
+
+R
R
RR
R
4
3
2
1R
R
R
R=
10
1
4
3
2
1== R
R
R
R
kΩ5,2
31 == RR
kΩ25
42 == RR
)(10)( tvtv io =
13
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
Standard instrumentation amplifier
high Ri
very good common mode rejection ratio
OA1 and OA2:
-high input resistance
- set the gain
OA3:
-gain = 1
- conversion from two voltages (vO1 and vO2) to a
single voltage (vO)
-additional rejection of the common mode
14
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
Standard instrumentation amplifier
2
1
2
1
1
2
1
1
IIO
v
R
R
v
R
R
v−
+=
1
1
2
2
1
2
2
1
IIO
v
R
R
v
R
R
v−
+=
( )
21 OOO vv
R
R
v−=
( )
21
1
2
2
1IIO vv
R
R
v−
+=
15
Laura-Nicoleta IVANCIU,
Electronic devices
Differential amplifiers
Differential amplifier
C9 – Summing and differential amplifs w/ OpAmp
Integrated precision differential amplifiers
•AD8221 Analog Devices
•MAX4194, MAX4195, MAX4196, MAX4197
Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation
Amplifiers Maxim Integrated
•LT1167 Linear Technology
Precision Instrumentation Amplifier
Av = 1 + (49.4 kΩ/RG)
Common uses of instrumentation amplifiers: sensor readings for
medical and industrial applications. Examples?
16
Laura-Nicoleta IVANCIU,
Electronic devices
Recap –circuits with OpAmp
Recap –circuits with OpAmp
Given a circuit with OpAmp, how can we tell whether the
circuit is:
inverting or non-inverting?
a simple comparator, a hysteresis comparator, or an amplifier?
What parameters do we compute, for each of the above? What
can we tell about the output voltage?
C9 – Summing and differential amplifs w/ OpAmp
17
Laura-Nicoleta IVANCIU,
Electronic devices
Recap –circuits with OpAmp
Recap –circuits with OpAmp
C9 – Summing and differential amplifs w/ OpAmp
Type of feedback vI goes
to Application We
compute vO
No feedback +Simple comparator,
non-inverting VTh vO∈{VOL ; VOH}
-Simple comparator,
inverting
Positive feedback +Hysteresis comparator,
non-inverting VThL
VThH vO∈{VOL ; VOH}
-Hysteresis comparator,
inverting
Negative feedback +Amplifier, non-inverting AvvO∈(VOL ; VOH)
-Amplifier, inverting
18
Laura-Nicoleta IVANCIU,
Electronic devices
Summary
Today’s menu consisted of a fine selection of OpAmp
circuits, such as:
Summing amplifiers with OpAmp
Inverting summing amplifier
Non-inverting summing amplifier
Differential amplifiers with OpAmp
Recap –circuits with OpAmp
Next week: Applications with OpAmp
To do: Homework 7
C9 – Summing and differential amplifs w/ OpAmp